1. Field of the Invention
The present invention relates to an imaging optical system and an image reading apparatus using the same. The present invention is particularly preferable in reading information on an image such as a monochrome image or a color image by using a line sensor of an image scanner, a digital copier, a facsimile, or the like, which uses a compact imaging optical element having various aberrations corrected in a well-balanced manner with high resolution.
2. Related Background Art
Up to now, a flat-bed type image scanner has been known as an image reading apparatus (image scanner) for reading image information on an original surface.
The conventional flat-bed type image scanner has read the image information in such a way that an imaging lens and a line sensor are fixed therein to move only a reflection mirror, thereby subjecting the original surface to slit exposure scanning.
On the contrary, in recent years, a carriage integral type scanning system has often been employed, which integrates a mirror, an imaging lens, a line sensor, and the like in order to simplify an apparatus structure, and scans an original surface.
FIG. 15 is a schematic diagram showing a main part of a conventional image reading apparatus of a carriage integral type scanning system. In the figure, light flux irradiated from an illumination light source L directly illuminates an original O placed onto an original table glass CG, an optical path of the reflected light flux from the original O is bent inside a carriage C via a first reflection mirror M1, a second reflection mirror M2, and a third reflection mirror M3 in order, and the light flux is imaged on a surface of a line sensor LS by an imaging lens (imaging optical system) 4. Then, the carriage C is moved in a direction of an arrow A (sub-scanning direction) shown in FIG. 15 by a sub-scanning motor B, so that the image information of the original O is read. The line sensor LS of FIG. 15 is constituted by arranging plural light receiving elements in a one-dimensional direction (main scanning direction).
FIG. 16 is an explanatory view showing a basic structure of the image reading optical system of FIG. 15.
In the figure, reference numeral 4 denotes an imaging optical system; IR, IG, and IB, line sensors for reading image information of colors R (red), G (green), and B (blue), respectively; and OR, OG, and OB, reading ranges on an original surface corresponding to the line sensors IR, IG, and IB, respectively. The carriage C scans the stationary original surface in the image reading apparatus shown in FIG. 15. Here, the scanning of the carriage C is equivalent to movement of the original surface O with respect to the stationary line sensor LS and the imaging lens 4 as shown in FIG. 16. By scanning the original surface O, an identical part can be read by the line sensors of different colors at a certain time interval. At this time, as shown in FIG. 15, in the case where the imaging lens 4 is composed of an ordinary refraction system, an axial chromatic aberration and a chromatic aberration of magnification occur. Thus, defocus or positional deviation take place in line images to be formed on the line sensors IB and IR as compared with the reference line sensor IG. Therefore, when the respective color images are superimposed to reproduce an object to be scanned, color bleeding or deviation is conspicuous in a resulting image. That is, there arises a problem in that requirements cannot be met regarding performances of a high aperture ratio and a high resolution.
On the other hand, recently, it has been clarified that, even in a decentered optical system, it is possible to establish an optical system in which aberrations are corrected sufficiently by introducing the concept of a reference axis to make constituent surfaces thereof asymmetry and aspherical. For example, a designing method for such an optical system is disclosed in JP 09-5650 A and design examples thereof are disclosed in JP 08-292371 A and JP 08-292372 A. Further, disclosed in U.S. Pat. No. 6,522,475 B, U.S. Pat. No. 5,999,311 B, U.S. Pat. No. 6,313,942 B, and U.S. Pat. No. 6,459,530 B are magnification optical systems using the above optical system.
FIG. 14 is a sectional view partially showing a main part of a reflection optical system as disclosed in JP 08-292371 A. In FIG. 14, the reflected light flux from the object passes through the diaphragm and enters a reflection optical element B1. In the reflection optical element B1, the light flux is refracted at a surface R1, reflected by surfaces R2, R3, R4, R5, and R6, and then refracted at a surface R7 and emitted from the reflection optical element B1. In the optical path, the light flux is primarily imaged on an intermediate image plane around the second surface, thereby forming a pupil around the fifth surface. Then, the light flux emitted from the reflection optical element B1 is finally imaged on an image pickup surface (image pickup surface of an image pickup medium such as CCD).
In the structure of FIG. 14, the optical element having the plural curved reflection surfaces and flat reflection surfaces integrated therein is used, thereby downsizing the entire mirror optical system. At the same time, the reflection mirrors are arranged with high precision irrespective of the mirror optical system.
Also, the diaphragm is arranged closest to the object side in the optical system and an object image is formed at least once in the optical system. Therefore, regardless of the reflection type optical element with a wide field angle, an effective diameter of the optical element can be reduced. The plural reflection surfaces constituting the optical element are applied with an appropriate refractive power and arranged in a decentering manner. As a result, the optical path in the optical system is bent into a desired shape and the whole length of the optical system in a predetermined direction can be shortened.
Such a decentered optical system is called an off-axial optical system. The off-axial optical system includes an off-axial curved surface as a curved surface, in which when the axis extending along reference light beam passing through an image center and a center of the pupil is assumed as a reference axis, a surface normal line of an optical surface constituting the optical path at a crossing point with the reference axis is not on the reference axis. In the off-axial optical system, the reference axis has a folded configuration at each off-axial curved surface. With the off-axial optical system, since constituent surfaces thereof are generally decentered and eclipse never occurs even on the reflection surface, it is easy to establish an optical system using the reflection surface.
On the other hand, when the reflection surfaces constitute the optical system, the decentering error is generally easy to occur. Thus, if the reflection surfaces are used to constitute the optical system, each reflection surface should be maintained with precision. Further, as another conceivable problem, in the case where the curvature radius of the reflection surface is small and the curvature is large, the decentering error is likely to occur. Also, the larger the distance between the surfaces after the reflection, the larger the positional deviation of the light beam due to the error of the surface inclination.
An original reading system such as a digital copier is difficult to constitute by the integral type optical system because a high resolution and high-speed performance are required therefor. Since an imaging lens required in the reading system needs to be bright and have a high resolution, it is difficult to increase a field angle in order to assure an optical performance. If the field angle is small, such a problem that the resultant optical path length increases is caused.
Further, in the case where a color image is read, as the resolution of the imaging lens becomes higher, the optical performance is more affected by a difference of imaging positions for the respective colors due to the chromatic aberration or by a chromatic aberration such as color drift in a screen. Therefore, the optical system is required, which can reduce the occurrences of the chromatic aberration.